Thermal protection for shielded ignition systems



M. F. PETERS 2, 86, 3

v THERMAL PROTECTION FOR SHIELDED IGNITION SYSTEMS Jan. 9, 1940.

Filed Dec. 7, 1954 3 Sheets-Sheet l INVFNTOR .F. PfTERS.

ATT RNEY Jan. 9, 1940. M. F. PETERS THERMAL PROTECTION FOR SHIELDEDIGNITION SYSTEMS Filed Dec. '7, 1934 3 Sheets-Sheet 2 INVENTOR M.PETERS. BY ATTZNEY .1 4

M. F. PETERS ,186,03 THERMAL PROTECTION FOR SHIELDED IGNITION SYSTEMSJan. 9, 1940.

Filed Dec. '7, 1934 3 Sheets-Sheet 5 INVENTOR MF Pt'TERS.

ORNEY.

" Patented Jan. 9, 1940 UNITED STATES THERMAL PROTECTION FOR SIIIELDEDIGNITION SYSTEMS Melville F. Peters, Riverdale, Md.

Application December '7, 1934, Serial No. 756,431

62 Claims.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 3700. G. 757) This invention relates to spark plugs and has for its objectthe provision of means so to control the transmission and dissipation ofheat from the plug to prevent the destructive heating of the in- 5sulation on thecable connected thereto and so to control the temperatureof those portions of the center electrode and the insulating materialthereon exposed to hot gases inthe combustion chamber of an internalcombustion engine as to prevent carbon fouling, preignition, and rapidwear of the sparking points of the plugs.

In the drawings:

Fig. 1 is a longitudinal section of a spark plug and means forrestricting the flow of heat therefrom to the cableshielding harness,the plug being provided with cooling fins to dissipate heat;

Figs. 2, 4, 5, and 6 show different modifications of the means forpreventing the transfer of excessive heat to the shielding harness ofthe cable;

Figs. 3, 7, and 8 illustrate certain new features in the construction ofthe spark plug;

Fig. 9 is an enlarged detailed view of the sealing means employed in theconstruction shown in Fig. '7;

Figs. l0, l1, and 12 illustrate my novel construction of the sparkingpoints carried by the spark plug shell;

Figs. 13 and 14 illustrate devices for connecting two parts of anelectrical circuit.

The use of radio on aircraft necessitates shielding the spark plugs andignition, cables of the engines of the craft to prevent the radiation ofelectrical disturbances that interfere with satisfactory operation ofthe radio. This has resulted in much difficulty from destruction of theinsulation on the cables by the high temperature due to heat conductedfrom the spark plug by the metal shielding harness of the plug and thecable. The prevention of such destruction of the insulation is one ofthe principal features of my invention. f

It is an elementary principle of heat conduction that if a poor thermalconductor be connected in heat transferring relation to a body havingr'elapoor conductor remote from the good conductor tively high thermalconductivity, the end of the nomenon is the basis of some importantaspects of the present invention.

Referring to Fig. 1, the shell l8 of the "spark plug is externallythreaded at I9 to engage the engine block and has a tubular member 20engaged with its outer portion. It is to be understood that the termsouter" and "inner" refer to the parts of the spark plug with referenceto their relative positions with respect to the combustion chamber ofthe engine. Center electrode 2| is carried by tubular member 20 andextends into the shell l8 to position its inner end in proper relationto the sparking points 22 on the shell. The so-called cigarette 2| is amica sheath around electrode 2|. Mica washers 23 are slipped overelectrode 2| and are suitably held under compression by a brass ferrule24 contracted upon the mica sheath 2| and pressed into the lower portionof tubular member 20. A second group of mica washers 25 is seatedagainst the outer end of ferrule 24 and against these abut the micawashers 26 that are tapered with outwardly diminishing cross section,the washers 25 and 26 being held under compression by a cap'21 fixed onthe outer extremity of electrode 2|. A sleeve 28 of copper, silver, orother metal which is a better conductor of heat than is centerelectrode. 2|, is fitted upon centerelectrode 2| and extends from apoint adjacent theinner end of the electrode over a substantial portionof the length ofthe electrode, this sleeve being externally coated withchromium, nickel or other metal that is resistant to oxidation, ithaving been found that the leakage of gas from the combustion chamberalong the electrode quickly oxidizes copper in this position unlessprotected as specified. If found preferable, a core of highly conductivemetal may be' used in the center electrode either instead of the sleeve28 or in combination therewith. An axial bore 29 extends. from the outerend of electrode 2| through a substantial portion of the length of theelectrode, preferably to a point near the outer end of sleeve 2|, theouter end of bore 29 being filled preferably asindicated at 29' toprevent collapse of hollow end. A cylindrical sleeve of mica is placedwithin the tubular member 20.

Owing to the high thermal conductivity of the sleeve 28, the heatabsorbed by the center electrode 2| is rapidly conducted outwardly toferrule 24 and the lower portion of tubular member 20 whence it istransferred to the cooling fins 2| which dissipate it to the air. Thediminished cross section of metal .in theouter portion of the electrode,due to the bore 28, reduces the thermal conductivity of this part of thecenter electrode and tends to prevent conduction of large quantities ofheat thereby.

It is well known that if that portion of the center electrode within theopen portion of the shell, and hence exposed to the hot gases in thecombustion chamber, is kept too cool, carbon will quickly accumulate andshort circuit the plu thus preventing its proper functioning, whereas ifthese parts reach a temperature that is too high they will ignite thecharge in the combustion chamber prematurely and cause prelgnition.Heretofore it has been the dictum in thisart .that a short plug is acool plug and a long plug is a hot plug, the length in question used inconstructing the center electrode must also be considered and that thecorrect relation of these'factors' may -be expressed. generally by theratio wherein Z is the length of the electrode and insulation exposed tothe hot gases and k is the coeflicient of thermal conductivity of thecenter electrode. Thus the length 1 may, within limits, be given anydesired value provided It also has such value as to give their ratio therequired magnitude.

It will be apparent that when the above described plug is used withoutshielding the poor thermal conductivity of the portion thereof beyondthe shell will prevent excessive cooling of the portion exposed to thegases in the combustion chamber and thus greatly decrease the tendencyto foul, while overheating of the inner portion of the center electrodeis prevented by the conduction of heat to the outer end of the shell.

Connected to the threaded portion 32 adjacent the outer end of tubularmember by a union coupling 33 is a hollow member 34 of material that iselectrically conducting but is a poor conductor of heat, whichconstitutes a unit in the electrical shielding, but cuts down greatlythe transfer of heat to the cable shielding which is connected theretoby means of a member on the shielding harness that engages the threads35.

' An example of a poor thermal conductor is nichrome but many othermetals which are poor conductors and are easier towork are on themarket. The union 33 seats against a shoulder 38 from which acylindrical flange 31 extends outwardly, this flange being adapted toslip over inserted. A wire 4| of nichrome or other 'elec-- tricallyconducting material that is a poor conductor of heat, is connected tocontact members 42 and 43, here shown in-the form of springs, that makeelectrical connection with the cable and the center electrode of thespark plug, respectively. It is tobe understood that the configuration.of member 34 may be changed to adapt it to the location in which it isto be used and it is not necessarily of the curved form shown in Pig. 1

v shield 81.

It may be seen'that this construction embodies a portion having goodheat conductivity connected in heat transferring relation to a memberhaving poor heat conductivity and thus the shielding. harness on cable40 will receive relatively little heat from the spark plug and theinsulation on the cable will be unimpaired regardless of the operatingtemperature of the spark plug. Actual test has proved the correctness ofthis. The extension of insulating material 34 into tubular member 20very notably increases the flash over distance in this part of theassembly.

Fig. 2 shows the application of the present invention to one well knowntype of spark plug wherein the center electrode 44 extends beyond thetubular member 45. The member 34 is secured to the upper portion 46 ofthe tubular member and the insulation 38 therein is recessed to receivethe electrode 44 and the cap 41 thereon which contacts the member 43that is connected to wire 4|.

In Fig. 3 the center electrode 48 is reduced in cross section over a potion of its th n a it outer end as shown at 49. Brass ferrule is shrunkor pressed upon the electrode 48 and cigarette 5 i, around the saidportion of reduced cross section, and thereby effects a gas-tight sealbetween the several parts, the ferrrule being pressed into the enlargedportion 52 of hollow member 53, the cylindrical body 54 of heat andelectrical insulating material being provided with a wire 4| that isconnected to electrode 48 by a member 43.

In Fig. 4 the heat and electrical insulation 54 extends beyond thehollow member 55 of stainless steel or other material having likeproperties, into the tubular member 58 of the spark plug. Throughout themajor portion of its length in the member 55 the insulating material 54is hollowed to receive the insulated cable 40 which is connected tocenter electrode 51 by members 42 and 43 and the wire 4|. The cableshielding harness 58 is connected to member 55 Ma union 3!. A

form of heat insulating connection adapted for I of member 59 and has init a small recess 83 into which may be passed a connecting element tocontact a suitable contact element 64. Coupling serves to connect theassembly with the cable shielding harness.

Spark plug 66 in Fig. 6 is .enclosed by an electrically conductingshield 41 in which is an electrical insulator member 6! that extendssome distance inwardly past the outer end of plug 64.. the member andthe shield 61 having aligned apertures through which the connectingelement may be led to the spark plug 45. in improvement on thisstructure lies in placing between the shield-and the insulation a member89 that is a poor conductor of heat and is in the form of a discwithcylindrical flanges l0 and II projecting therefrom throughtheabove-mentionedapertures in the insulation 48 and in the Through theopening thus defined there is passed a cylindrical element 12 that isboth thermally and electrically insulating and in which is a wire 4| oflow thermal conductivity that is electrically connected to the sparkplug. 86 by the member 13, a hollow cylinder 14 of stainless steel orthe like being fitted over the member 12 and cylindrical flange 19.

Fig. 7 illustrates a novel method of sealing the spark plug against gasleakage. The mice. cylinder 16 is so formed as to produce a shoulder 19adjacent the outer end 01' center electrode 2| and the space outwardlyfrom said shoulder, between the electrode and the mica cylinder 19, isfilled with a metal that has considerable ductility when cold, such as ametal foil for which copper has been found suitable, this filling beingindicated by reference character IT. The ferrule 18 is then contractedupon the mica cylinder and the electrode, either by shrinking or bypressing, and the sealing body 11 is firmly gripped and held inposition. This construction is shown on a larger scale in Fig. 9. Theferrule I8 is thereafter pressed into the inner enlargedportion oftubular member I9. Another method of efiecting a seal at the outer endof the center electrode is to force foil 80 between center electrode 2|and mica cylinder I5 or to impact copper therein, place a ring 9| aroundthe outer end of the center electrode to seat against the sealing body80 and then upset the outer end of the electrode to retain the ring andthe sealing body in position, as shown at 11', Fig. 9. a

Instead of pressing or shrinking a solidbushing I ll upon the centerelectrode and the mica cylinder therearound, it is perhaps preferable toassemble the cylinder in tubular member 19 and place a mandrel thereinto hold the mica cylinder in shape, the space between the cylinder 15and the lower portion of tubular member 19 being then filled withimpacted granular metal or-metal foil, while the metal or foil is eitherhot or cold. After the mandrel is withdrawn, center electrode 2| may beassembled therewith and fixed in position by upsetting the outer endthereof as above explained in connection with Fig. 8.

Long study has shown that the rate of wear,

of the sparking points 22 that are fixed to the shell it, due to hightemperature and electric discharge between these points and the centerelectrode, increases very rapidly as the temperature of the pointsrises. The life of these points 22 is greatly increased by making themwith a core 92 of copper or other metal that is a good conductor of heatenclosed in a shell of the metal or alloy generally employed for suchpoints, the copper serving to conduct the heat rapidly to shell l9 andthe outer coating 83 of the more resistant material protecting thecopper from excessive wear. Fig. 10 shows the preparation of theimproved type of sparking points, the rod 94 of spark plug pointmaterial having the coppercore 92 completely enclosed therein at eachend with an intervening body 89 of the usual point material. This isthen shaped as shown in Fig. 11 and cut through the portion 85, afterwhich the points may be secured to the spark plug shell I! in anysuitable manner.

Heretofore some dimculty has been experienced in securing an efiicientelectrical connection between the several parts of the ignition circuitwhere springs such as 42 and I3, Fig. l, or the like, have beenemployed. To eliminate this trouble I have devised a new means of makingsuch connection. In Fig. 13 is shown a hollow member 88 of metal that isa poor thermal conductor having in it the heat and electrical insulation81 with the wire I9 01' low heat conductivity embedded therein, therebeing arecess 99 formed in the insulating material II to receive thecable 99. Fixed to the wire 9| in cable I are a plurality of curvedfingers 92 that are adapted to be spread apart when pushed against thebulbous terminal 93 on wire 88 to slip around and engage said terminal.To prevent casual separation of the fingers 92 from terminal 99 a sleeve94 is slidably positioned on the cable and after the fingers are engagedwith the terminal this sleeve is pushed down to clamp the fingersagainst the terminal. At the outer end of sleeve 94 is a flange 95 thatextends radially outwardly over the end of insulating material 91 andagainst which the shielding harness 96 is clamped by a union 91, a body98 of asbestos, relatively soft rubber or the like, disposed in a thinmetal retainer 99 being so positioned that when con-' nection 91 istightened down the rubber will be squeezed tightly around the cable 90and against the adjacent end of sleeve 94 to form a tight seal and toform an electrically conducting connection, through retainer 99, betweenmembers 89 and 96. The fiange 95 may be caught with the fingers or ascrew driver to pull sleeve 94 outwardly when it is desired todisconnect the cable from the terminal 93. The sleeve 94 may be slit atits inner end or it may be inwardly bevelled or otherwise shaped toincrease its effectiveness for the purpose stated. In Fig. 14 thefingers 92 are shown provided with outwardly turned portions 92' attheir free ends. These outwardly turned portions contact the walls ofrecess 89 and so center the fingers 92 in proper position to engage theterminal 93.

The invention herein set forth may be made or used by or for theGovernment of the United States without payment of any royalties thereonor therefor.

I claim:

1. In combination, a spark plug, comprising a shell, a substantiallytubular member engaged with the outer end of said shell, a centerelectrode mounted in said member to extend through said shell to theinner end thereof, said electrode having an external copper sleevethereon extending from adjacent the inner end of the electrode to apoint outwardly of sa d shell and an axial bore extending from adjacentthe outer end oi said sleeve to the outer end of the electrode, saidsleeve being externally plated with chromium, and a member of lowthermal conductivity having internal and external" electricallyconducting portions, said internal conducting portion being connectibleto said electrade and to a cable and said external portion beingconnectible to said tubular member and to an electrical shieldingharness on said cable.

with the outer end of said shell, a center electrode mounted in saidmember to extend through said shell to the inner end thereof, saidelectrode having an external sleeve of high thermal conductivity thereonextending from adjacent the inner end of the electrode and an axial boreextending from adjacent the outer end of said sleeve to the outerend ofthe electrode, said sleeve being of a length that is a substantialfraction of the length of said electrode and externally plated with ametal that is resistant to oxidation; and a member of low thermalconductivity having internal and external electrically conductingportions, said internal portion being connectible to said electrode andto a cable and said external portion being connectible to said tubularmember. and to an electrical shielding harness on said cable.

3.11 combination, a spark plug, comprising a shell, a substantiallytubular member engaged with the outer end of said shell, a centerelectrode mounted in said member to extend through said shell to theinner end thereof, said electrode having an external sleeve of highthermal con} ductivity thereon extending from adjacent the inner end ofthe electrode, said sleeve being of a length that is a substantialfraction of the length of said electrode and externally plated with ametal that is resistant to oxidation; and

. a member of low thermal conductivity having 'trode and said member,said cylinder'having a shoulder adjacent the outer end of saidelectrode, and a sealing body of metal that has a substantial degree ofplasticity when cold forced in between said electrode and said cylinderagainst said shoulder, the outer end of said electrode being upset toretain said body in position.

5. A spark plug, comprising a shell, a tubular member engaged with theouter end of said shell, a center electrode mounted in said member, aninsulating member interposed between said electrode and said tubularmember, said insulating member having a shoulder adjacent the outer endof said electrode, and a sealing body of metal that has a substantialdegree of plasticity when cold forced in between said electrode and saidinsulating member against said shoulder, the outer end of the electrodebeing upset to retain said body in position.

6. A spark plug, comprising a shell, a tubular member engaged with theouter end of said shell, a center electrode mounted in said member, aninsulating member interposed between said electrode and said tubularmember, said insulating member having a shoulder adjacent the outer end01' said electrode, and a sealing body of metal, foil impacted betweensaid electrode and said insulating member against said shoulder, theouter end of said electrode being upset to retain said body in position.

'7. A spark plug, comprising a shell, a tubular member engaged with theouter engLof said shell, a center electrode mounted in said member, aninsulating member interposed between said electrode and said tubularmember, said insulating member having a shoulder adjacent the outer endof said electrode, and a sealing body of metal, foil impacted'betweensaid electrode and said. insulating member against said shoulder.

8. A spark plug, comprising a shell, a tubular member engaged with theouter end of said shell,

a center electrode mounted in said member, an

insulating member interposed between said elecinsulating member againstsaid shoulder, the

outer end of said electrode being upset to retain said body in position.

9. A spark'plug, comprising member engaged with the outer end .of saidshell, a center electrode mounted in said member, an

a shell, a tubular insulating member interposed between said electrodeand said tubular member, said insulating member having a shoulderadjacent the outer end 01 said electrode, and a sealing body of copperimpacted between said electrode and said insulating member against saidshoulder.

10.' A spark plug, comprising a shell, a tubular member engaged with theouter end of said shell, a center electrodemounted in said member,'aninsulating member interposed between said electrode and said tubularmember, said insulating member having a shoulder adjacent the outer endof said electrode, and a sealing body between said electrode and saidinsulating member seated against said shoulder.

11. A spark plug, comprising a shell, a tubular member engaged with theouter end of said shell, a center electrode mounted in said member, asleeve of copper around said electrode extending over a substantialportion of the length of said electrode, said sleeve being externallycoated with a metal that is resistant to oxidation, a cylinder of micainterposed between said electrode and said member, said cylinder havinga shoulder adjacentthe outer end of said electrode, and a sealing bodyof metal impacted between said electrode and said cylinder against saidshoulder.

12. A spark plug, comprising a shell, a tubular member engaged with theouter end of said shell,

a center electrode mounted in said member, a

sleeve being externally coated with a metal that is resistant tooxidation, a cylinder of mica interposed between said electrode and saidmember, said cylinder having a shoulder adjacent the outer end of saidelectrode, and a sealing body of metal impacted between said electrodeand said cylinder against said shoulder.

13. A spark plug, comprising 'a shell, a tubular member engaged with theouter end of said shell, a center electrode mounted in said member, asleeve of metal having a high heat conductivity around said electrodeextending over a substantial portion of the length of said electrode,said sleeve being externally coated with a metal that is resistant tooxidation, an insulating member interposed between said electrode andsaid tubular member, said insulating member having a shoulder adjacentthe outer end of said electrode, and a sealing body of metal impactedbetween said electrode and said insulating member against said shoulder.

14. A spark plug, comprising a shell,,a tubular member engaged with theouter end of said shell, a center electrode mounted in said member, asleeve of metal having a high heat conductivity around said electrodeextending over a substantial portion 01' the length of said electrode,said sleeve being externally coated with a metal that is resistant tooxidation, an insulating member interposed between said electrode and.said tubular member, and sealing means between said electrode and saidinsulating member adjacent the outer end of said electrode.

15.,A spark plug, comprising a shell, a center eectrode mounted therein,said electrode being. of diminished cross section over a portion of itslength adjacent its outer end, a sheath of mica around said electrode,and a metal ferrule constricted upon said sheath and said electrode overthe portion of reduced cross section.

l6. A spark plug, comprising a shell, a center.

electrode mounted therein, a sheath of mica.

2,1eo,ose

around said electrode, and a metal ferrule constricted upon said sheathand said electrode over a portion of its length adjacent the outer endof the electrode.

17. A spark plug, comprising a shell, a center electrode mountedtherein, said electrode being of diminished cross section over a portionof its length adjacent its outer end, a sheath of mica around saidelectrode, a body of insulating material around said sheath extendingfrom the inner edge of said diminished portion to adjacent the inner endof said electrode, and a metal ferrule constricted upon said sheath andsaid electrode over the said portion to hold said insulating materialunder compression.

18. A spark plug, comprising a shell, a center electrode mountedtherein, a sheath of mica around said electrode, a body of insulatingmaterial around said sheath extending from adjacent the inner end of theelectrode over a substantial portion of the length of the electrode, anda metal ferrule constricted upon said sheath and said electrode adjacentthe outer end of the electrode to hold said insulating material undercompression.

19. A spark plug, comprising a shell, a center electrodemounted'therein, a sleeve of metal having high heat conductivity aroundsaid electrode over a substantial portion of its length, said sleevebeing externally coated with a metal that is resistant to oxidation, a.sheath of insulating material around said electrode, said sheath havingan internal shoulder adjacent the outer end of said electrode, a body ofinsulating material extending from adjacent the inner end of theelectrode over a substantial portion of the length of the electrode, aferrule of metal constricted upon said sheath and said electrodeadjacent the outer end of the electrode and seated at one end againstsaid body of insulating material, and a sealing element of impactedmetal between said electrode and said sheath abutting against saidshoulder.

20. A spark plug, comprising a shell, a center electrode mountedtherein, a sheath of mica around said electrode, said sheath having aninternal shoulder adjacent the outer end of said electrode,- a sealingbody of impacted metal between said electrode and said sheath abuttingagainst said shoulder, and a metal ferrule constricted upon said sheathand said electrode adjacent the outer end of the latter. V

21. In a spark plug, a center electrode whereof a substantial portionbeginning adjacent the inner end thereof has a relatively high heatconductivity and the remainder has an abnormally low heat conductivity.

22. A spark plug, comprising a shell, 8. member engaged with the outerend of said shell and forming an extension thereof, a center e ectrodecarried by said member and extending into said shell, said electrodehaving a portion of relatively high heat conductivity beginning adjacentthe inner end of the electrode and extending well intosaid member andthe remainder of the electrode having a very much lower capacity forheat conduction, and cooling fins carried by said member in a positionto receive heat by conduction from the said high conductivity portion ofthe electrode.

23. A spark plug, comprising a shell, a member engaged therewithand'forming an extension thereof, a center electrode carried by saidmember, and cooling fins lying in radial planes of said electrodecarried by said member in a position to receive heat by conduction fromsaid electrode.

24. The combination with a spark plug, having a shell, a tubular memberengaged with the outer end thereof, a center electrode carried by saidmember, and on said electrode adjacent its outer end a body ofinsulating material having an outwardly diminishing cross section, of ahollow member made of electrically conducting material of low heatconductivity, said hollow member being engageable at one end with saidtubular member and at the other end with an electrical shielding harnesson a cable, insulating material in said hollow member projecting fromthe first mentioned end thereof to extend into said tubular memberaround the insulating material on said electrode and at the other endprovided with a cavity to receive the end of said cable, an electricallyconducting wire of low heat con ductivity extending longitudinallythrough said insulating material in the hollow member, and means at eachend of said wire to form contacts with said cable and said electrode,respectively.

25. The combination with a spark plug having a shell, a tubular memberengaged with said shell and having adjacent its outer end an externallythreaded portion of greater diameter than adjacent parts of the member,and a center electrode assembly carried by said member that is at itsouter end radially spaced from said member, of a hollow metal member oflow thermal conductivity having at one end a radially extending shoulderand an outwardly extending cyli'ndri cal flange adapted to seataroundthe outer end of said tubular member, an internally threadedconnecting element seated against said shoulder, and engageable with thesaid threaded portion of the tubular member, electrically insulatingmaterial of low thermal conductivity in said hollow member projectingrom the said one end of the hollow member tc extend'into said tubularmember around a portion of said electrode assembly, there being in theother end of said insulating material a cavity for receiving the end ofa cable, a wire of low thermal conductivity passing longitudinallythrough said insulating material, and means at each end of said wire toconnect electrically said wire to said electrode and said cable,respectively.

26. An element of an electric circuit that includes a spark plug and anelectrically shielded insulated cable, comprising an electricallyconducting member of low heat conductivity. securable at one end to thespark plug and at the other end to the cable shielding harness,insulating material of low heat conductivity in said member, anelectrically conducting wire of low thermal conductivity enclosed insaid insulatof low thermal conductivity in said member pro-- jectingfrom the end of said member that is connectible to said tubular portionto extend into said tubular portion and having at its other end a cavityto receive the end of a cable, a wire oif low thermal conductivityextending through of said wire to form contacts with the wire of thesaid cable and with said electrode, respectively.

28. The combination with an electrically shielded spark plug and ashielded cable associated therewith, of an electrically conductingelement of low thermal conductivity interposed between.

the shielding of the plug and the shielding of the cable, and connectingmeans between the center electrode of "said plug and the wire of saidcable, said means including a body of insulating material of low thermalconductivity and afwire passing through said insulating material, saidwire being likewise of low thermal conductivity. 29. The combinationwith a spark plug having a center electrode, and a shielded cableassociated therewith, ofmeans having low thermal 'thermal conductivityconnecting said electrode conductivity electrically connecting said plugto the shielding on the cable and means of low to the wire in saidcable.

- 30. A spark plug, including a center electrode sodesigned andconstructed that the ratio 1 1/ has'a predetermined and selected value,Z being the length of the center electrode and insulating materialthereon exposed to gases in the combustion chamber and lc being thecoeflicient of thermal conductivity of the saidelectrode.

31. A spark plug, including a shell, and acenter electrode whereof thematerial is so chosen and the length thereof within the open portion ofthe shell is so adjusted that the ratio J? has a selected value, 1 beingthe said length of the electrode within the shell and k being thecoefficient of thermal conductivity of the material of said electrode.

32. A spark plug, comprising a shell having transversely inwardlyextending, heat dissipating electrode projections, each of. saidprojections having a core of copper.

33. A spark plug, comprising a shell having transversely inwardlyextending, heat dissipating of material having a high thermalconductivity.

36.- A spark plug, comprising a shell, having a portion engageable withan engine body, a center electrode carried thereby, the portion of saidelectrode within the shell having high thermal conductivity and theportion of the electrode beyond the outer end of saidshell having anabnormally low thermal conductivity, and an element forming, a path ofhigh thermal conductivity from I said electrode tothe portion ofsaidshell beyond said engagingportion.

37. In a spark plug, a center electrode whereof the longitudinally outerportion has a notably said insulating material, and means at the endslower thermal conductivity than does the inner portion thereof.

38. In a spark plug, a center electrode whereof substantially thelongitudinally outer half has a notably lower thermal conductivity thandoes the remainder thereof.

39. In a spark plug, a center electrode whereof the outer portion inexcess of one-fourth the length thereof and not substantially more thanone-half said length has, a notably lower thermal conductivity than doesthe remainder thereof.

40. The combination with a spark plug, of'

members having low thermal conductivity associated therewith to preventradiation of electrical disturbances therefrom, and heat radiat ing finscarried by one of said members.

41. The combination with a spark plug, of members associated therewithto prevent radiation of electrical disturbances therefrom, andsubstantially plane heat radiating fins lying in radial planes of. saidspark plug carried byone of said members.

42. In a spark plug, a center electrode having an appreciably higherelectrical conductivity in its inner portion than in the outer portionthereof.

43. In a spark plug, a center electrode having an appreciably higherthermal conductivity in its inner portion than in the outer portionthereof.

44. In comb-ination with a cylinder head, a spark plug and a cabletherefor, a chrome-nickel alloy shield to impede the transmission ofheat from the cylinder head and the spark plug to the cable.

45. In combination with a spark plug and a cable therefor terminating ata point removed from the spark plug, an electrical conductor. of lowheat conductivity in electrical contact with the cable and partiallybridging the distance be-' tween the spark plug and the cable andcontact means to connect the spark plug and the conductor.

46. In combination with a cylinder head, a I

spark plug and a cable therefor, a shield of metal of low heatconductivity to impede the transmisthereof formed of a metal of low heatconductivity to restrain the transmission of heat from the lower portionof the shield to the upper portion thereof.

50. In combination with a spark plug and a cable therefor terminating ata point removed from the spark plug, an electricalconductor of low heatconductivity connecting the spark plug and the cable.

51. In combination with a spark plug, a cable therefor and a shieldingspark plug cap provided withan aperture which normally receives 9. ca.-ble, 8. rod of metal of low heat conductivity disposed in the apertureof the cap in contact with the spark plug and a cable holding meansaddecent the aperture to maintain the cable in in spaced relation to thespark plug, said rod and cable being electrically connected.

52. In combination with a spark plug, a cable therefor and a shieldingspark plug cap, a rod of metal of low heat conductivity disposed in thecap, a sleeve carried adjacent the cap, said sleeve being provided witha bore to receive the cable .and spring means to'maintain the sparkplug, rod and cable in electrical contact. 10 53. In a spark plug shieldincluding a dielectric cap disposed over a spark plug, a shield of metalof low heat conductivity to restrain the transmission of heat to thedielectric cap.

54. In a spark plug shield including a dielectric cap disposed over aspark plug, spark plug contact means carried by the cap, said contactmeans being made of metal of low heat conductivity.

55. In combination with a spark plug and a cable therefor terminating ata point removed from the spark plug, an electrical conductor of low heatconductivity in electrical contact with the cable and partially bridgingthe distance between the spark plug and the cable.

56. As a new article of manufacture a rod of metal of low heatconductivity and a covering of dielectric material on the rod, said rodbeing adapted to be disposed in the electrical circuit between a sparkplug and a spark plug cable.

57. In combination with a radio shielded spark plug cable and a radioshielded spark plug cap which supports spark plug contact means, anadapter to connect the shielding of the cap and cable comprising asleeve of dielectric material provided with a bore, a metallic couplingmember on the outside of the sleeve in electrical contact with theshielding'of both the cap and the cable and means to electricallyconnect the cable and the spark plug contact means in the cap.

58. In combination with a radio shielded spark plug cable and a radioshielded spark plug cap which supports spark plug contact means, anadapter to connect the shielding of the cap and cable comprising asleeve of dielectric material 5 provided with a bore, a metalliccoupling member on the outside of the sleeve in electrical contact withthe shielding of both the cap and the cable, and a metallic member oflow heat conductivity connecting the cable and the spark plug con- 10tact means in the cap.

59. In a spark plug, a heat dissipating ground electrode consisting of aplurality of circular metallic members, the member on the inside of theassembly having higher heat conducting' qualities than those on theoutside.

60. In a spark plug, a heat dissipating ground electrode consisting of alaminated body circular in form and comprising a plurality of metalsheets in contact with one another, the center 20 sheet being copper andthe sheet on either side being nickel alloy.

61. A heat shield for spark plug cables comprising an electricalconductor of low heat conductivity and a covering of dielectric materialon 25 the conductor, said conductor being disposed in the electricalcircuit between a spark plug and a spark plug cable.

62. A heat shield for spark plug cables comprising an electricalconductor of low heat conductivity, a covering of dielectric material onthe conductorand radio shielding means surrounding the conductor andinsulation, said conductor being disposed in the electrical circuitbetween a spark plug and a spark plug cable.

MELVILLE F. PETERS.

